The goal of truly integrating computational capacity with the everyday lives of individuals has consumed substantial research effort, and from a variety of perspectives. Through component miniaturization, wearable items (such as wristwatches, jewelry, and clothing accessories) can now offer computer processing power to perform a variety of tasks while remaining casually and continuously available. Recent advances in fabric design permit clothing itself to carry electrical signals, while electrodes distributed around a an individual's environment can silently and unobtrusively monitor position, orientation and movements. In these ways, processing capacity can be dispersed over various worn components, and the user can, without conscious effort, interact computationally with the surrounding environment.
An important principle of design for wearable devices is adaptation to users' existing habits and preferences, rather than forcing the user to adapt to accommodate new appliances. The concept of "personal area networks" (PANs), which utilize the user's body as an electronic communication channel, represents a significant step in this direction. As described in U.S. Pat. No. 5,914,701, electrostatic coupling among worn or carried electronic devices allows these items to intercommunicate via the user's body, sharing data or control signals among themselves, or transferring data to an external recipient (such as another person or a wall-mounted receiver) by close proximity or touching.
Electrostatic coupling represents a departure from traditional forms of electronic communication, which involve radiated energy. For example, radio-frequency identification (RFID) devices have been employed for some time to remotely sense parameters of interest in people or objects. An RFID device receives a wireless signal from an externally located "reader," which determines the parameter of interest based on the response of the RFID device to the transmitted signal. A simple application of this technology is security: an individual wears an RFID "tag" or badge, and a controlled-entry system unlocks a door only if the wearer's tag is recognized as s/he approaches.
Radiative systems can be configured for a relatively large (i.e., far field) read range. But this capability can actually represent a disadvantage if the environment contains multiple, independent RFID devices, since the reader will excite all devices within the read range simultaneously. Proximately located devices, in other words, cannot share the same frequency channel; separate addressing requires separate frequencies or cumbersome efforts to focus the electromagnetic field from the reader.
Magnetostatic and electrostatic RFID systems, by contrast, operate through near-field interaction, and thereby facilitate selective coupling or "channel sharing"; that is, so long as the tagged items are not immediately adjacent (i.e., within a few centimeters of each other), they can be individually addressed. In terms of selectivity, electrostatic systems offer practical advantages in terms of the ease of focusing an electric field as compared with a magnetic field. Electrostatic systems also offer manufacturing and cost advantages, since the induction coil required for magnetostatic systems is eliminated and electrodes can be conveniently and inexpensively deposited on substrates of widely varying shapes and materials. For example, the tag in a magnetostatic system may have a coil with 100-1000 turns and a radius of 1-5 cm, while a typical reader has a 20-cm coil.
On the other hand, a person's body can act as a shield in electrostatic systems, compromising the coupling between reader and RFID device. The PAN approach, which uses the entire body as a signal carrier, not only overcomes this limitation, but also substantially extends the read range. For example, a computer housed in a user's shoe can readily communicate electrostatically with the user's wristwatch, personal digital assistant, and/or notebook computer.
One limitation of the PAN concept has been the need for autonomously powered devices. Again, components such as batteries or power supplies add significantly to the weight and cost of PAN devices, and contradict the goal of seamless integration of computational capacity into the user's lifestyle and habits.